Heretofore, an exposure apparatus has been widely used for transferring a fine circuit pattern onto a wafer to produce an integrated circuit in the lithography technique. According to the progress in achieving high integration, high response speed and high performance of integrated circuits, miniaturization of the integrated circuits has progressed and the exposure apparatuses are requested to form an image of a circuit pattern on a wafer surface with a long focal depth and with high resolution, and use of an exposure light source emitting shorter wavelength has been in progress. As the exposure light source, besides the conventional g-line (wavelength 436 nm), i-line (wavelength 365 nm) and KrF excimer laser (wavelength 248 nm), ArF excimer laser (wavelength 193 nm) is starting to be employed. Further, in order to deal with next-generation integrated circuits having a circuit line width of at most 100 nm, F2 laser (wavelength 157 nm) is regarded as a predominant candidate for the exposure light source, but this light source is considered to cover only until the generation of 70 nm line width.
In such a technical trend, as a next generation exposure light source, a lithography technique using EUV light (extreme ultra violet light) attracts attention since it is considered to be applicable for plural generations of 45 nm and after. EUV light means light in a wavelength band in a soft X-ray region or in a vacuum ultra violet region, and specifically, light having a wavelength of from 0.2 to 100 nm. Currently, as a lithography light source, use of 13.5 nm has been studied. The principle of the exposure in this EUV lithography (hereinafter referred to as “EUVL”) is the same as the conventional lithography in that a mask pattern is transferred by employing a projection optical system. However, since there are few materials transmitting light in the energy region of EUV light, it is not possible to use a refraction optical system, and a reflection optical system has to be employed instead. (Refer to JP-A-2003-505891)
A mask to be used for EUVL is basically constituted by (1) a glass substrate, (2) a reflective multi-layer film formed on the glass substrate, and (3) an absorptive material layer formed on the reflective multi-layer film. As the reflective multi-layer film, one having a structure that a plurality of materials having different refractive indexes at the wavelength of the exposure light, periodically laminated with the period in the order of nm, is employed, and Mo and Si are known as the typical materials. Further, as the absorptive layer, Ta and Cr are studied. As the glass substrate, a material having a low thermal-expansion coefficient is required so as not to have deformation even under irradiation of EUV light, and a glass having a low thermal-expansion coefficient or a crystallized glass have been studied. The glass substrate is produced by polishing such a glass or crystallized glass material with high precision and cleaning.
Further, as a method for polishing a surface of a work such as a glass or a metal to have a small surface roughness (smoothness), JP-A-8-120470 (with reference to paragraphs [0007] to [0011]) describes a method of precisely polishing a mechanically polished surface of a work having a predetermined surface roughness, by a gas-cluster ion beam. In this method, by irradiating the surface of the work with accelerated gas-cluster ions, the gas-cluster ions irradiated are destroyed by collision with the surface, and at this time, the ions collide with molecules or atoms constituting the work, and those molecules or atoms flow in the lateral direction with respect to the surface of the work, whereby the surface of the work is cut in the lateral direction. In the JP-A-8-120470 mentioned above, a metal mold, a glass substrate, a ceramic substrate or the like is exemplified as the work and argon, nitrogen gas, oxygen gas, carbon dioxide gas or the like is exemplified as the source gas for the gas-cluster ions.